-LRB- CNN -RRB- -- As Mitt Romney prepares to make the case for a first term , give a thought to what we can expect from a Barack Obama second term .

It 's probably going to look a lot like the second terms of George W. Bush , Bill Clinton , Ronald Reagan , and Richard Nixon -- driven not by his own agenda , but by events beyond his control . Bush 's second term was defined by the after-effects of the Iraq decision he made in his first term ; Clinton 's by impeachment and the Internet boom ; Reagan 's by deficits and Gorbachev ; Nixon 's by Watergate .

And what will define Obama 's ?

1 -RRB- Republicans will hold at least one House of Congress , maybe two .

2 -RRB- The U.S. economy will continue to mend , as the financial crisis recedes into the past .

3 -RRB- Unless forcibly prevented , Iran will enrich enough uranium for multiple nuclear weapons sometime before 2017 .

4 -RRB- Natural gas production will continue to surge .

Prediction 1 implies that there will be no bold Obama legislative agenda in a second term . The votes just wo n't be there . The second-term Obama agenda will have to be executed through agency action and executive order . And he 'll have lots of opportunities via the big laws passed in his first term : the Affordable Care Act , Dodd-Frank , and so on .

Critics often talk about how long these laws are . Yet however long the enactment , the text of a complex modern statute is only the very roughest guide to its future operation . The crucial operating decisions are made by presidential appointees , supervised by the courts . Congress drafts , but the administrators fill in the details - and the details , as the saying goes , are where the devil lurks .

A re-elected President Obama will have large powers to reshape the health industry and financial services -- not with some dramatic law , but with a thousand undramatic lower-level edicts that cumulatively can redefine a law in ways never imagined by its authors .

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But there is one agreement that the president and Congress will have to reach : over future taxes and budgets . The Bush tax cuts of 2001 , extended in 2010 , will expire at the end of 2012 . The president has said he would allow those tax cuts to lapse , at least for high income levels . Republicans are determined to preserve them . Who 'll blink ? On past precedent , you might guess that the president will . But the president knows that people expect him to blink -- which may inspire him to hang much , much tougher .

2 -RRB- Recovery comes much more slowly from recessions caused by financial crises , according to the authors of the best study of the subject , Carmen Reinhart and Kenneth Rogoff .

Yet if recovery comes slowly , it does come eventually . By 2015 , the U.S. economy should be reaching normal growth and fuller employment . Deficits will begin to shrink ; the country 's present desperate mood will abate . And Republicans and Democrats will furiously argue over who deserves the credit , as they still argue over who deserves credit for the 1990s boom .

We also saw in the 2000s , however , that employment can grow strongly without generating very much in the way of wage increases for most working people . The U.S. economy added almost 2 million jobs in 2004 , more than 2 million in 2005 , about 1.7 million in 2006 , and more than 700,000 in 2007 . Yet the median wage as of the end of 2007 still had not caught up to its 2000 peak , adjusting for inflation .

In the post-2009 recovery , almost all the income growth has been captured by the most affluent few . President Obama may deplore that trend , but he seems to have no very clear idea of how to alter it .

3 -RRB- U.S.-led sanctions and Israeli-led sabotage have slowed the Iranian nuclear program . Few experts expect that these non-belligerent means will work indefinitely .

What then ? Israel could attack Iran . The United States could hit even harder . But suppose -- as seems most likely -- that both allies flinch from war . Then the U.S. and Israel will confront a radically reshaped Middle East , one in which America 's friends will be weaker and more fearful and Iran will be more assertive and intransigent .

The president 's hopes of reconciling the Islamic and Western worlds -- of midwifing a Palestinian state -- and of finishing off jihadist terrorism will all bump into the reality of an emboldened Iran and depleted American prestige .

4 -RRB- Second terms tend to bring disappointments , but one huge success seems to be gathering for whoever happens to occupy the White House over the next four years : the advent of that long promised goal , energy independence .

The surge in natural gas production in the United States , the full development of the massive oil sand resource in Canada , could make North America the world 's fastest-growing energy region in the years ahead and put the continent on the way to self-sufficiency by 2020 . It wo n't have quite happened yet by 2017 , but the achievement will at least be visible on the near horizon .

It will be a strange irony of history if the president of hope and change left office as the president who presided over the greatest surge in domestic fossil fuel production since the 1930s .

The opinions expressed in this commentary are solely those of David Frum .

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David Frum : A second term for Barack Obama would have a different shape

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He says Dems likely wo n't control both houses , giving him little chance for major legislation

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Frum : Obama would likely benefit from an improving economy but would face challenges overseas

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U.S. energy production boom would bring U.S. closer to energy independence , he says